Low level threshold detector with temperature compensation



' Jan. 17, 1967 R. c. BULLENE 3,299,296

LOW LEVEL THRESHOLD DETECTOR WITH TEMPERATURE COMPENSATION OriginalFiled June 1, 1962 W 45 DVCOLTS R2 n FIG I T FIG 2 THERMISTOR ALONECOMBINATION T SENSISTOR ALONE 3 0C 25C [00C I N VEN TOR. ROBERT C. BLjlLENE Y M ATTORNEYS United States Patent This is a continuation ofapplication Serial Number 200,030, filed June 1, 1962, by Robert C.Bullene, now abandoned. I I This invention relates in generalto aswitching circuit.

and in particular to a trigger circuit iwhich switchesfrom a first stateof conduction to a second state on the application of a signal voltageand returns to the initial condition when the signal voltage falls belowa preset level.

It is often desirable to indicate the presence or absence of a signaLas,for example, a direct current voltage, and the present invention iscapable of producing'an output by switching a transistor circuit from afirst to a second conductivestate on the application of a direct currentsignal. It is also capable of being adjusted so as to switch back to theinitial condition when the signal voltage falls 'below a preset level. Av

It is a feature of this invention to provide a switching circuit ofgreat sensitivity which is temperature compensated by a novel circuit'soas to allow the voltage level adjustments to remain substantiallyconstant with variations in temperature.

It is an object of this invention to provide for a switching circuitcapable of switching from a first to a second condition on theapplication of a signal. Another object of this invention is to providea temperature compensating circuit for a switching device.

Further objects, features, and advantages of the invention will becomeapparent from the following description and claims when read in view ofthe drawings, in which:

FIGURE 1 is a schematic view of the switching circuit of this invention;

FIGURE 2 is a detailed view of the temperature compensating circuitry;and

FIGURE 3 is a plot of resistance versus temperature for variouscomponents used in the circuit.

FIGURE 1 illustrates an input terminal 10 to which may be connected apositive direct current voltage. The

input terminal 10 is connected to the base 11 of a transistor Q1 throughresistor R5. Resistor R6 is connected between base 11 and ground. Theemitter of transistor Q1 is connected to movable contacts 12 and 13which engage, respectively, resistors R1 and R2. One end of resistor R2is connected to ground. Resistor R1 is connected in series with resistorR12 through a diode D2 to ground. Resistor R12 is also connected throughresistor R11 to a negative direct current bias supply which forillustrative purposes is shown in the drawing as being minus 45 voltsdirect current.

A resistor R4 is connected to the emitter of transistor Q1 and is inseries with a thermistor RT] which has its other side connected toground. The collector 14 of transistor Q1 is connected to the base 16 ofa second transistor Q2 through a diode D1.

The emitter 17 of transistor Q2 is connected to a resistor R3 which hasits opposite side connected to movable contacts 12 and 13. The collector18 of transistor Q2 is connected through a resistor R10 to a positivebias source.

A temperature compensating circuit is connected between the collector 14and 18 of transistors Q1 and Q2 and comprises a resistor R7 which isconnected to the parallel circuit consisting of a thermistor RT3 whichis in series with a resistor R8 and a sensistor RT2. A resistor R9 isconnected between resistor R10 and the parallel.

circuit of sensistor RT2 andthe thermistor RT3 and resistor R8. Thejunction point between the "resistors R8 and.

R9 is connected to-ground. -A pair. of output terminals 19 and 20connected cross ,resistorR3l Y t 7 There. area variety of-semiconductorpositive action circuits -.'now in use that operate at various signallevels and. voltagefinputs. Devices such as trigistors, unijunctiontransistors, silicon controlled rectifiers and. transistors have beenused in a variety of circuits. Despite the numerous designs, none has apositive action that will trigger at low level positive direct currentand return to. the original state at a direct current level above zero.A low level direct current detector at, lower voltage levels is verydifiicult to temperature stabilize due to the inherent characteristicsof transistors. I A circuit which has a positive action and istemperature compensated is shown' in FIGURE 1. "Under no signal{condition, Q1 is cut off and Q2 is conducting at saturation. Thisfstajtemaybe designated Mode ,1. When a positive direct current signal isapplied at the input of Q1, the circuit snaps to another modejofoperation (Mode 2). in this mode,fQ1.is conducting and Q2 is cut 01?.Lowering the signal Voltage to apreviously ad-' justed leve'lwillcausethe circuit tojrvert to its original state. T The adjustment ofsignal voltage'at which the circuit snaps to Mode 2 is made withcontact12'on R1; the

adjustment of signalvoltage at which the circuit returns to Mode 1 ismade with contact 13 on R2. j The a'dju'st-. ment of R1 simply changesthe bias voltage onfth'e emitter of Q1, and therefore changes thethreshold signal level. The common resistance for both emittersdetermines the signal differential voltage between Mode 1 and Mode 2;

Due to circuit current flow in the common emitter resistance, thevoltage differential decrease as the emitter resistance decreases.

The thermistors RTl and RT3 and sensistor RT2 push the ambient'operatingtemperature from 0 C. to near C. with'very little change in directcurrent threshold. The largest share of the compensation is due to RT2and RT3. The temperature compensation network shown in FIGUREZhas theassociated compensation curve shown in FIGURE 3. my

Thisis the direct curve, since the leakage current of transistorsincreases exponentially with temperature. At room temperature and below,the leakage current of silicon diode D is so small that there is nochange in collector current. Therefore, below 25 C. no compensation isneeded. The desired result is accomplished with a sensitor andthermistor.

The ability of the circuit to operate at low direct current (50milliwatts and up) is one of the biggest advantages. Also, it ispossible to adjust for signal voltage threshold and voltage differentialbetween operating modes. The circuit can be made compact enough to beused where space is at a premium. Of course, the temperaturecompensation from 0 to 80 C. and the positive action of the circuit arealso advantageous.

The novelty of this circuit is the ability to operate with one polaritydirect current signal voltage at very low level and still maintain apositive action circuit.

This circuit can be applied wherever a variable direct current thresholdlevel is needed for control of relays or other circuits.

FIGURE 2 shows the temperature compensating circuit, and it is to benoted that the total resistance across terminals 21 and 22 produces atemperature characteristic such as shown in FIGURE 3 by the solid line,and the net effect of the resistance variations with temperature acrossterminals 21 and 22 is to compensate for the exponential temperaturevariation in the transistors Q1 and Q2. Thus the sensistor andtransistor produce temperature compensation.

In order to adjust the switching point of the circuit the movablecontact 12 is adjusted to a suitable level until switching occurs. 50 asto switch back upon the decrease of voltage applied to terminal 10,movable contact 13 may be adjusted.

The output at terminals 19 and 20 will bea voltage which is zero whentransistor Q2 is not conducting and transistor Q1 is conducting. Thiscondition exists when a positive direct current voltage has been appliedto terminal 10. When this signal has been removed or has fallen below apredetermined level, Q1 will stop conducting and transistor Q2 willcommence conducting and a voltage will be developed across the resistorR3 which is other than zero.

Although the invention has been described with respect to a particularembodiment thereof, it is not to be so limited, as change-s andmodifications may be made therein whichare within the spirit and scopeof the invention as defined by the appended claims.

I claim:

1. 'A switching circuit for low level threshold detection, comprisingfirst and second transistors each having a base, a collector and anemitter, a first impedance means common to the emitter circuits of bothsaid transistors, means for applying a voltage between the collector andthe emitter of said second transistor to render said second transistornormally conducting, means for applying across said first impedancemeans a voltage of a polarity opposite to that produced in said firstimpedance means bythe flow therethrough of the emitter current of saidnormally conducting second transistor, means for applying an inputsignal to the base of said first transistor to thereby render said firsttransistor'conducting, means interconnecting the collector of said firsttransistor and the base ofsaid second transistor to apply the voltage ofthe former to the latter to render said second transistor nonconductingupon the occurrence of conduction in said first transistor, secondimpedance means in the emitter circuit of said second transistor acrosswhich an output signal is developed, and a temperature compensatingcircuit in the collector circuit of said first transistor including athermistor and a sensistor in parallel therewith.

2. A switching circuit comprising first and second transistors eachhaving a base, a collector and an emitter, a variable bias voltage,means connecting said transistors to said variable bias voltage, a fixedbias voltage, said fixed bias voltage rendering said second transistornormally conducting, an input terminal for applying an input voltage tosaid first transistor, said input voltage rendering said 4 firsttransistor conductive, an interconnection between the collector of saidfirst transistor and the base of second transistor for rendering saidsecond transistor nonconducting when said first transistor isconducting, and a temperature compensating circuit cooperating with saidfirst transistor to compensatefor the nonlinear temperature response ofsaid transistor, said temperature compensating circuit comprising athermistor and a sensistor in parallel.

3. The switching circuit'of claim 2 including a second thermistor inparallel with said variable bias voltage.

4. The switching circuit of claim 2 including a resistor in series withsaid thermistor.

5. A switching circuit comprising a first normally nonconductivetransistor and a second normally conductive transistor, an inputterminal to said first transistor for rendering it'conductive uponapplication of an input signal, means connecting the collector of saidfirst transistor to the base of said second transistor for renderingsaid second transistor nonconductive when said first transistor isconductive and also for returning said first and second transistors totheir normal states when said input signal is removed, variable biasmeans connected to the emitters of said first and second transistors,and a temperature compensating circuit including a thermistor and asensistor in parallel for compensating for the nonlinear temperatureresponse of said transistors.

6. A switching circuit comprising a first and a second transistor, theemitters of said transistors being connected to a first bias means, thecollectors of said transistors being connected through a temperaturecompensating circuit and a first resistor in series with saidcompensating circuit, said compensating circuit comprising a thermistorand a second resistor in series, and a sensistor inparallel With saidthermistor and said second resistor.

7. A temperature compensating circuit comprising a thermistor and asensistor in parallel. V

8. The circuit of claim 7 including a first resistor in series with saidthermistor.

9. The circuit of claim 8 including a second resistor in series with theparallel combination of said thermistor and said sensistor.

References Cited by the Examiner UNITED STATES PATENTS 2,914,685 11/1959McVey 30788.5

FOREIGN PATENTS 557,559 11/1943 Great Britain.

ARTHUR GAUSS, Primary Examiner.

J. BUSCH, Assistant Examiner.

1. A SWITCHING CIRCUIT FOR LOW LEVEL THRESHOLD DETECTION, COMPRISINGFIRST AND SECOND TRANSISTORS EACH HAVING A BASE, A COLLECTOR AND ANEMITTER, A FIRST IMPEDANCE MEANS COMMON TO THE EMITTER CIRCUITS OF BOTHSAID TRANSISTORS, MEANS FOR APPLYING A VOLTAGE BETWEEN THE COLLECTOR ANDTHE EMITTER OF SAID SECOND TRANSISTOR TO RENDER SAID SECOND TRANSISTORNORMALLY CONDUCTING, MEANS FOR APPLYING ACROSS SAID FIRST IMPEDANCEMEANS A VOLTAGE OF A POLARITY OPPOSITE TO THAT PRODUCED IN SAID FIRSTIMPEDANCE MEANS BY THE FLOW THERETHROUGH OF THE EMITTER CURRENT OF SAIDNORMALLY CONDUCTING SECOND TRANSISTOR, MEANS FOR APPLYING AN INPUTSIGNAL TO THE BASE OF SAID FIRST TRANSISTOR TO THEREBY RENDER SAID FIRSTTRANSISTOR CONDUCTING, MEANS INTERCONNECTING THE COLLECTOR OF SAID FIRSTTRANSISTOR AND THE BASE OF SAID SECOND TRANSISTOR TO APPLY THE VOLTAGEOF THE FORMER TO THE LATTER TO RENDER SAID SECOND TRANSISTORNONCONDUCTING UPON THE OCCURRENCE OF CONDUCTION IN SAID FIRSTTRANSISTOR, SECOND IMPEDANCE MEANS IN THE EMITTER CIRCUIT OF SAID SECONDTRANSISTOR ACROSS WHICH AN OUTPUT SIGNAL IS DEVELOPED, AND A TEMPERATURECOMPENSATING CIRCUIT IN THE COLLECTOR CIRCUIT OF SAID FIRST TRANSISTORINCLUDING A THERMISTOR AND A SENSISTOR IN PARALLEL THEREWITH.